The invention relates to an oil scraper piston ring for internal-combustion engines and is of the type which has a one-piece ring body whose runner face is subdivided into three partial runner faces Radially throughgoing oil outflow apertures are provided between adjoining partial runner faces
Up-to-date internal-combustion engines are conventionally provided with pistons which have a first and a second compression ring and an oil scraper ring. Thus, three axially spaced circumferential grooves have to be provided an the piston face for accommodating the three piston rings. The oil scraper piston rings used for this purpose are of steel or cast iron and have an R or X-shaped cross-sectional configuration so that on the circumferential surface of the piston ring oriented towards the cylinder two runner webs are formed whose outer surfaces function as runner faces. A circumferential groove for receiving a hose spring is provided in the radially inner surface (back surface) of the piston ring. Such a piston ring is disclosed, for example, in German Patent No. 42 00 489.
For improving the scraping function of the oil scraper piston ring, among others the tangential force of the hose spring may be utilized.
The force of the spring affecting the runner webs generates, in case of small sliding faces, a relatively large specific area pressure. Since, however, the wear of the slidingly engaging components increases as the area pressure increases, the service life of such rings is reduced and with increasing wear a deterioration of the scraping action occurs.
To improve the oil scraping effect, it is conventional to provide the runner faces of the runner webs with a wear-resistant coating and also, to increase the number of webs. German Patent No. 10 65 235 discloses an oil scraper piston ring which has three runner webs whose outer faces engage the inner face of the cylinder wall.
In case more than two runner webs are present in the ring structure, however, the engineering problem is presented that during engine operation, because of the temperature-caused distortions, not all the runner faces lie simultaneously on the cylinder wall. As a solution of such a problem, conventionally only one or two web faces have been provided with a wear-resistant coating. The web without a wear-resistant protective coating has been dimensioned with a radial excess, so that the uncoated web, during operation, wears down in such a manner that after a certain time period all the webs lie against the inner cylinder surface.